It was a bright April day in Washington, D.C., when a large crowd gathered on the South Lawn of the White House to greet a man celebrating his 81st birthday.

The president of the United States introduced him. Then he gave a brief address -- still viewable on C-SPAN -- that focused on the greatness of America.

Pope Benedict XVI understood this nation's vision.

"From the dawn of the Republic, America's quest for freedom has been guided by the conviction that the principles governing political and social life are intimately linked to a moral order based on the dominion of God the Creator," said this pope on that day in 2008.

"The framers of this nation's founding documents drew upon this conviction when they proclaimed the 'self-evident truth' that all men are created equal and endowed with inalienable rights grounded in the laws of nature and nature's God," he said.

Benedict made clear his belief that the American people had not abandoned these principles in the more than two centuries that had passed since our founding.

"The course of American history," he said, "demonstrates the difficulties, the struggles, and the great intellectual and moral resolve which were demanded to shape a society which faithfully embodied these noble principles.

"In that process, which forged the soul of the nation, religious beliefs were a constant inspiration and driving force, as for example in the struggle against slavery and in the civil rights movement," he said.

"In our time too, particularly in moments of crisis, Americans continue to find their strength in a commitment to this patrimony of shared ideals and aspirations," he said.

The belief that all governments and all men must abide by the God-given natural law -- which, as Benedict saw it, was expressed in the Declaration of Independence and was evident in American history -- is also found in the Catechism of the Catholic Church.

"The natural law, present in the heart of each man and established by reason, is universal in its precepts and its authority extends to all men," says the Catechism. "It expresses the dignity of the person and determines the basis for his fundamental rights and duties."

The Catechism continues: "The natural law, the Creator's very good work, provides the solid foundation on which man can build the structure of moral rules to guide his choices. It also provides the indispensable moral foundation for building the human community."

Just as Benedict saw the abolition of slavery and the civil rights movement within the United States as causes rooted in the natural law, his predecessor, St. John Paul II, saw the Cold War struggle against Communism as fundamentally a moral conflict.

In that conflict, John Paul II was allied with the American President Ronald Reagan, who had unapologetically declared the Soviet Union an "evil empire."

Freedom, Benedict argued in his speech on the South Lawn, "is a challenge held out to each generation, and it must constantly be won over for the cause of good.

"Few have understood this as clearly as the late Pope John Paul II," he said. "In reflecting on the spiritual victory of freedom over totalitarianism in his native Poland and in eastern Europe, he reminded us that history shows, time and again, that 'in a world without truth, freedom loses its foundation,' and a democracy without values can lose its very soul.

"Those prophetic words," said Benedict, "in some sense echo that conviction of President Washington, expressed in his Farewell Address, that religion and morality represent 'indispensable supports' of political prosperity."

Benedict said: "Democracy can only flourish, as your founding fathers realized when political leaders and those whom they represent are guided by truth and bring the wisdom born of firm moral principle to decisions effecting the life and future of the nation."

Now -- just as in 1963 or 1763 -- the greatest challenges to freedom in America come when government acts against the immutable truth of natural law.

These days, injustice in this nation manifests itself in such things as the legalized killing of unborn babies, legalized marriage and adoption by same-sex couples, and in edicts meant to force people to act against their consciences in cooperating in what they know to be wrong.

If America retains its founding commitment to a national order rooted in natural law, we will remain free. If we don't, we won't.